In a digital radio communication system, such as a cellular telephone and a car telephone, which has drastically become very important in recent years, a base station set up in each cell allocates radio communication channels to a plurality of communication terminals existing in a cell and at the same time communicates by radio with the above terminals.
FIG. 1 is a view showing a configuration of a digital radio communication system. It is supposed in FIG. 1 that base stations 11 through 13 are set up in cells 21 through 23, respectively. Mobile stations 31 through 33 are supposed to be located in the above cell 21 at present, and to be under radio communication with the above base station 11.
FIG. 2 is a view showing details of signals received at the above mobile station 31 in FIG. 1. Though a signal transmitted from the base station 11 is a desired signal S for the mobile station 31 under radio communication with the base station 11, there is included noise N as shown in FIG. 2, when the above transmitted signal is received at the mobile station 31.
Moreover, a received signal R comprises as an interference signal I, as well as the above noise N: an interference signal Iintra, which is transmitted from the base station 11 to mobile stations 32, 33 except the own station, in a cell having a base station currently under communication (hereinafter, called as “own cell”); and an interference signal Iinter, which is transmitted from the base stations 12, 13 being not the communication ends, in other cells.
A receiving apparatus installed in the mobile station 31 performs automatic gain control (hereinafter, called as “AGC”) of the received signal; demodulates the desired signal included in the received signal after conversion into a digital signal; and fetches a received data. Here, AGC is control of the electric field strength of the received signal to a predetermined target value in order to improve the accuracy at digital conversion of the received signal.
Hereinbelow, a configuration of a conventional receiving apparatus installed in a mobile station will be described, using a block diagram of FIG. 3.
In the receiving apparatus of FIG. 3, a receiving RF section 52 amplifies a radio frequency signal received through an antenna 51, and performs frequency conversion of the amplified signal into a baseband one. An AGC section 53 controls according to a gain coefficient, the gain of the above baseband signal output from the receiving RF section 52. An analog-to-digital conversion section 54 coverts the output signal of the AGC section 53 into a digital signal.
A despreading section 55 multiplies the output signal from the above analog-to-digital conversion section 54 and the same spreading code as that of the transmitting side. An interference canceller 56 cancels the interference signal from the output signal of the above despreading section 55. A demodulation section 57 demodulates the output signal of the above interference canceller 56; and a decoding section 58 fetches the received data after decoding the output signal of the above demodulation section 57.
An electric-field-strength-measurement section 59 measures the electric field strength of the output signal of the analog-to-digital conversion section 54. A gain-coefficient calculation section 60 calculates a gain coefficient so that difference between the electric field strength, which is measured at the above electric-field-strength-measurement section 59, and a target value is minimized. Here, a signal amplitude X (hereinafter, abbreviated as “amplitude X”), which may be expressed in terms of bit, and so on may be used as the above target value.
A digital-to-analog conversion section 61 converts the gain-coefficient calculated in the gain-coefficient calculation section 60 into an analog value for outputting to the AGC section 53.
Thus, the conventional receiving apparatus performs closed-loop AGC for improved accuracy at conversion of the received signal into a digital signal.
Here, the interference signal I is divided into the interference signal Iintra of the own cell and the interference signal Iintra of other cells, as described above. The above interference signal Iintra of the own cell includes desired signals of other users other than the pertinent user.
And, the low bit-accuracy in the above desired signal S causes deterioration in the interference-suppression effect, and the above interference-suppression effect is also deteriorated in the case of reduction in the bit accuracy of the interference signal Iintra of the own cell, when a MUD (Multi User Detection) type interference canceller is used as an interference canceller.
Accordingly, when ratios of the desired signal S, and the interference signal Iintra of the own cell included in the received signal R are small, the bit accuracy of the above signals becomes insufficient to cause deterioration in the received quality.
On the other hand, when AGC is performed so that clipping receiving is performed with the target value, which is set larger than the amplitude X, considering the interference signal Iintra of other cells and the noise N, the large ratios of the desired signal S and the interference signal Iintra of the own cell included in the received signal R causes clipping of even the above signals, and the received quality is deteriorated. Here, the clipping means that the peaks of signals and language syllables are cut out at transmission to the extent that detection is possible.
However, there is a problem, in the above conventional receiving apparatus, that there is a case where AGC cannot be performed with good accuracy, and the received quality is deteriorated, as the gain coefficient is calculated based on the electric field strength of the received signal without consideration of the ratios of each signal included in the received signal.